APIS information

My sister and niece live in the United States and hold American Passports, as well as Australian Passports. They will soon be travelling LAX/SYD/LAX. Their booking was made on the QF website and they are in the process of ensuring all is OK. Can you please advise - when it comes to entering their APIS information in their online booking, should they be entering their American or Australian Passport details? Many thanks, Gavin